Inexpensive solutions to excessive calcium in water? (Distilled water would be expensive over time)

depending how your humidifier works, RO / DISTILLED water may not work. The cold steam ones use ultra sonic waves to mist the water and some of them require minerals to be present in the water to work right. I also demineralise mine once a month/week with vinegar and and a mix of citric/asorbic acids. they eat away all that shit real fast. add in h202 to help break it up and clean the reservoir tank. The cold mist ones will always leave mineral coats on things that are static charged it just a after effect of them. I sometimes add in a splash of vinegar to the tank when its in use to help but it does make the house small like vinegar a bit. if its a wicking style or heat based one. just soak the wick / tank in CLR or same acid solution above. will take a bit but its how you get the caked on mineral crust lower. could also collect rain water and use that as an option. May want to boil it first to prevent any other things from growing in it, this is more of an issue with wicking style humidifier and smells comeing from the wick but ultrasonic versions also aerosolize water particles that bactera can hitch a ride on then you breath them in.